1. Field of the Invention
The present invention is directed generally to diagnostic equipment used to inspect manufactured goods and, more specifically, to diagnostic equipment used to
2. Description of the Prior Art
FIG. 1 illustrates a steam turbine blade 10 of the type typically manufactured from near-net shape, high alloy, stainless steel forgings. Typically, the type of steel used may be ASTM Type 403, 17-4PH, or the like. Depending upon the specific application, the blades 10 may range in size from about four inches (10.16 cm) in length and a few pounds in weight to four feet in length (1.22 m) and one hundred lbs. in weight. The larger blades are often used in large industrial steam turbines. A critical step in the production of a blade 10 is the final machining of the blade attachment area or root 12. That final machining operation involves multiple pass grinding with shaped abrasive wheels.
During the grinding operation, a grinding burn may occur as a result of a lack of cooling of the blade root or a dull abrasive wheel. A grinding burn is an area of blade material which has been heated sufficiently to change or degrade the metallurgical properties of the steel, and, in turn, oxidize the surface of the metal. Because of the multiple passes of the grinding operation, a burn which could have been detected visually as a dark spot (oxidized material) on the root surface is removed by the subsequent grinding pass leaving damaged metal but no surface oxidation.
Following the multiple grinding operation, the root 12 is examined both visually and with dye penetrant or magnetic particle nondestructive inspection methods to insure that no surface discontinuities exist in the root 12 of the blade. Particular attention is focused on the load bearing surfaces 14 and the lands or flats 16 therebetween that will eventually be in direct contact with the turbine disk attachment area i.e. the fir tree or steeple region of the disk. However, because oxidation may have been removed by a subsequent grinding pass, a visual examination after machining will not reveal any degradation.
Recently, eddy current inspection procedures have been developed to supplement the optical examination and, in particular, to enhance the ability to detect grinding burns. As a result of a grinding burn, blade material is turned from ferrite to martensite. The eddy current procedures can detect residual damage because that change in microstructure produces a significant change in eddy current signature. FIG. 2 shows the difference in eddy current response for a burned and unburned 403 blade material with a yield strength of 136 ksi. FIG. 3 shows the difference in eddy current response for a burned and unburned type 403 blade material with a yield strength of 120 ksi. As can be seen, in both cases the burned material results in an output signal having a substantially greater magnitude than the output signal produced by the unburned material. Thus, eddy current sensors can be used to detect residual damage as a result of grinding burns.
An eddy current examination of the root 12 of the steam turbine blade 10 can be conducted manually by using a pencil probe 18 of the type illustrated in FIG. 4. While pencil eddy current probes 18 of the type illustrated in FIG. 4 can detect and characterize grinding burn damage and other degradation in the root area of turbine blades, the procedures are slow and unreliable because the pencil probe 18 must be passed over all of the areas of interest thus making it likely that an area of interest will be skipped due to operator error. Thus, the need exists for an inspection device which is convenient, reliable, and can be used on a plurality of different blade roots and rotor steeples under production line conditions.